H. S. (Herbert Spencer) Jennings.

Contributions to the study of the behavior of lower organisms online

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directly into the lower organisms. But we are not confronted with the
alternative of doing this or of separating the two subjects completely.
The behavior of man can be studied from the same objective standpoint
which we employ in investigating the behavior of animals. When this
is done, there is no reason for holding the results on man aloof from
those obtained elsewhere ; if it is proper to compare different organ-
isms of any kind from this point of view, in order to obtain general
results, as all investigators do, it is certainly proper to draw man also
into the circle of comparison. The fact that in man we can know also
the subjective accompaniments of the different physiological states and
reactions is by no means a disadvantage in this comparison ; it is merely
an additional feature, of the highest possible interest. We can even,
it seems to me, justifiably call attention to the relation between the
subjective states as found in man to certain general phenomena common
to man and other organisms. It is only when we proceed directly to
attribute to the lower animals the subjective states which we know only
in man (and, indeed, only in our own individual minds) that we pass
the boundary of scientific procedure.

In the higher animals, and especially in man, the essential features
in behavior depend very largely on the history of the individual ; in
other words, upon the present physiological condition of the individual,
as determined by the stimuli it has received and the reactions it has
performed. But in this respect the higher animals do not differ in
principle, but only in degree, from the lower organisms, as we have seen
in our analysis of the behavior of Stentor. In this unicellular form
we were forced to distinguish at least six different physiological condi-
tions, determining in the same individual different reactions to the same
stimuli. In the higher animals, and especially in man, we can distin-
guish, as might be expected, an immensely greater number of such
conditions which induce different reactions, but there is no evident differ-
ence in principle in the two cases. Can we go farther and make a more
direct comparison of individual physiological states in the higher and
lower organisms? We find in Stentor, and again in the flatworm,
that after the organism has been repeatedly stimulated by an agent


which must in the long run be classed as injurious, it is thrown into a
physiological condition in which its reactions become more rapid and
powerful, and of such a nature as to remove the organism from the
source of stimulus. We find that in this state the organism reacts to
any stimulus to which it reacts at all by a strong negative reaction.
In higher animals we frequently find the same condition of affairs, and
the animal is then commonly said to be frightened. Finally, we often
find in man a similar condition, and here we know certain subjective
accompaniments of the physiological condition, the most characteris-
tic of which is perhaps the emotion of fear. In all these cases the
objective manifestations of the physiological condition are of the same
character. Does the fact that in man we know something additional
about the matter, the subjective accompaniments, constitute grounds
for denying the essential similarity, from a physiological standpoint,
of this condition in man and that in the lower organisms? It seems to
me that it does not ; in fact, all that is maintained in making the com-
parison is that this condition causes similar objective phenomena and
is brought about by similar conditions. Further than this our analysis
and comparison cannot go.

Another class of physiological conditions which we can distinguish
almost all the way through the animal series is that produced character-
istically by intense stimuli, as opposed to faint stimuli. As a rule, any
stimulus, even if it is one to which the organisms respond usually by a
positive reaction, produces, when it becomes very intense, reactions
whose general effect is to remove the organisms from the source of
stimulation (negative reactions).* This is true in Amoeba, where weak
mechanical stimuli cause spreading out and movement toward the
source of stimulus, while strong mechanical stimuli cause it to contract
and move away; it is true for Stentor ; it is true for the stimulus of
weak and strong light in Euglena and Volvox and many other
organisms ; it is true for mechanical and chemical stimuli in the flat-
worm ; it is true in general for higher animals and man. In all these
cases the intense stimulus evidently changes the physiological con-
dition so that the organism now reacts negatively. In man we know
that this physiological condition is accompanied subjectively by pain
or at least discomfort, and even in higher animals such reactions are
usually spoken of as pain reactions. Objectively considered, the
phenomena are analogous throughout the animal series, so that we

*"A11 organisms behave in two great and opposite ways toward stimulations;
they approach them or they recede from them. Creatures which move as a
whole move toward some kinds of stimulations, and recede from others. Crea-
tures which are fixed in their habitat expand toward certain stimulations, and
contract away from others." Baldwin, 1897, p. 199.


may properly characterize the physiological condition which produces
the negative reaction to strong stimuli, with Professor J. Mark Bald-
win (1897, p. 43) as the " physiological analogue of pain." This, of
course, by no means commits us to the belief that the organisms have
a sensation of pain ; concerning this we know nothing.

It thus seems to me possible to trace some of the physiological con-
ditions which we know, from objective evidence, to exist in man and
the higher animals, back to the lowest organisms. Many conditions that
we can clearly distinguish in man will doubtless be followed back to a
common single condition in the lower organisms ; but this is exactly what
we should expect. Differentiation takes place as we pass upward in
the scale, in these matters as in others.

The most interesting and important field in which we find the
behavior of higher organisms dependent on their previous history, and,
therefore, on their present condition as influenced by previous experi-
ence, is in that group of phenomena which we call memory, or learning
by experience. Memory has as its basis the general phenomenon that
a stimulus received or a reaction performed leaves a trace on the
organism, or modifies its condition in such a way that it later reacts
differently to the same stimulus. This basis of memory is, of course,
clearly present in Stentor.

The analysis of the different physiological conditions found in the
lower organisms, the influences to which they are due, and the
reactions of these organisms as influenced by physiological conditions
certainly forms a most promising field for research, and one as yet
almost untouched.


The present paper attempts to show, by an analysis of certain
phenomena in the behavior of lower organisms, taking Stentor and
Planaria as types, that physiological states of the organism are most
important determining factors in reactions and behavior. In these
organisms, to the same stimuli, under the same external conditions,
the same individuals react at different limes in radically different ways,
showing the existence of different physiological states of the organism,
which determine the nature of the reactions. In a unicellular organ-
ism (Stentor) we can distinguish at least six different physiological
states, in each of which the organism has a different reaction method,
and corresponding facts are brought out for the flatworm. Scattering
observations taken from works on tropisms, etc., are shown to indicate
that the same state of affairs is found in other lower organisms.

The conditions producing these different physiological states are
examined and their importance for the theory of behavior in the lower
organisms is brought out. The relations of these facts to " interference


of stimuli," ''heterogeneous induction," "spontaneous movements,"
and "changes in the sense of reactions with a change of intensity in
the stimulus," are developed.

The view is set forth that in most of the lower organisms a reaction
to stimulus usually involves the following factors: (i) the stimulus
changes the physiological state of the organism as a whole ; (2) this
change in physiological state induces a certain type of reaction.
Evidence for this view is summarized.

Finally, it is pointed out that realization of the importance of
physiological states as determining factors in the behavior of the lower
organism is of service in bringing the study of these organisms into
relation with that of higher animals and man. An objective study of
the behavior of these higher animals shows the prevalence of physio-
logical states as determining factors in behavior, and in some cases, at
least, some of these states are closely analogous to what we find even
in unicellular organisms.






Introduction: Objects of the Investigation 131

Description of the Movements and Reactions, 132

The Movements, I3 2

The Movements of Amoeba as described Formation and Retraction ofPseudopodia 152

by Rhumbler and Butschli ; Agree- Surface Currents in Formation of

ment with Currents in a Drop of Pseudopodia in contact with Sub-

Fluid Moving as a Result of a Local stratum 15*

Decrease in Surface Tension 137

Formation of Free Pseudopodia 153

Withdrawal ofPseudopodia. 156

Currents in Amoeba as studied from above; Movements at Anterior Edge 160

Lack of Backward Currents 134 Movements of Posterior Part of Body. ... 165

Movements of Upper and Lower Surfaces General View of Movements of Amoeba in

Studied Experimentally; Rolling Locomotion 169

Movement 138 Some Characteristics of the Substance of

Amoeba verrucosa and Its Relatives. 140 Amoeba 173

Other Species of Amoeba 146 Fluidity 173

Historical on Rolling Movements in Rhumbler's Ento-ectoplasm Process 173

Elasticity of Form in Amoeba 175

m<B a I4 Contractility in Ectosarc of Amoeba. 177

Reactions to Stimuli 181

Reactions to Mechanical Stimuli 181 Some Complex Activities 193

Positive Reaction 181 Activities connected with Food-taking 193

Negative Reaction 182 Taking Food _ 193

Reaction to Chemical Stimuli 187 Pursuit of Food 196

Reaction to Heat 190 Other Amoebae as Food 198

Reactions to Other Simple Stimuli 191 Reactions to Injuries 202

Physical Theories and Physical Imitations of Amoeboid Movements, . 204

Surface Tension Theory 204 Experimental Imitation of Movements

Berthold's Theory that One-sided Adher- due to Local Contractions of Ectosarc

ence to Substratum is the Cause of and of the Roughening of Ectosarc in

Locomotion 208 Contraction 215

Experimental Imitation of Locomo- Direct or Indirect Action of External

tion in Amoeba 209 Agents in Modifying Movements 219

Formation of Free Pseudopodia 214 Direct or Indirect Action in Food-taking, 222

General Conclusion 225

Behavior of Amceba from Standpoint of Comparative Study of Animal

Behavior, 226

Habits in Amceba 326 Relation of Different Reactions to Differ-

Classes of Stimuli to which Amceba Re- ent Stimuli; Adaptation in Beha-

acts 227 vior of Amceba 227

T of Reaction 227 Reflexes and "Automatic Actions" in

Amoeba 228

Variability and Modifi ability of Reactions 229

Summary, 230




The present paper contains the results of an investigation which was
undertaken with two general problems in mind. The first purpose
was to determine by observation and experiment, from the standpoint
of the student of animal behavior, how far recent physical and
mechanical theories go in aiding us to explain the behavior of Amoeba.
The second object of the work was to furnish needed additional data
on the reactions of Amoeba to stimuli, and to systematize and unify
our knowledge of its behavior.

The recent theories which would resolve the activities of Amoeba
largely into phenomena due to alterations in the surface tension of a
complex fluid seem to promise much. They are of precisely the
character from which most may be hoped ; from a study of the physics
of matter in a state similar to that found in the living substance, the
laws of action of this living substance are sought. Such theories have
been developed, as is .well known, by Berthold (1886), Quincke
(1888), Biitschli (1892), Verworn (1892), Rhumbler (1898), Bern-
stein (1900), Jensen (1901), and others. The success of this method
of attacking the problems seems great. Activities similar, at least
externally, to those of Amoeba, are produced by physical means, and
fully analyzed from the physical and mechanical standpoint. In this
manner the movement, the control of movement by external agents,
the feeding, the choice of food, the making of the shell, and other
features of the behavior have been more or less closely imitated,* and
in a way permitting a complete analysis in accordance with chemical
and physical laws.

From the standpoint of the student of animal behavior, the resolu-
tion of the behavior of any organism into the action of known physical
laws must be a matter of the deepest interest. The actions of higher
organisms seem at present so far from such a resolution that some
investigators believe an essential difference in principle to exist
between the behavior of living things and non-living things ; between
the laws of biology and those of physics. The resolution, then, of the
behavior of even the simplest organism into known physical factors
would be an event of capital significance, affecting fundamentally the
whole theory of animal behavior. A renewed thorough study of the

* See especially Rhumbler, 1898. 131


facts, with especial reference to these theories, seems, therefore, much
to be desired. The results of the present study will show, I believe,
that such a re-examination of the facts was greatly needed.

As to the second object of this investigation, stated above, it is a
somewhat remarkable fact that the observational basis for a number of
the most important reactions assumed to exist in Amoeba is exceedingly
scanty, particularly so far as control of the direction of movement is
concerned. For example, one of the reactions most often assumed to
exist in Amoeba, and most commonly selected for imitation by
physical means, is chemotaxis, the movement toward or away from a
diffusing chemical. But no account exists, so far as I have been able
to discover, of actual observation of such a reaction in Amoeba, under
experimental conditions. Again, the effects of slight or of intense
localized mechanical stimuli, in controlling the direction of movement,
has not been worked out in detail. To fill these and similar gaps in
our knowledge, and to bring the different reactions into relation with
each other, so as to make possible a connected account of the behavior
of Amoeba, is, then, the second object of this paper.

I shall first give an account of the movements and reactions of
Amoeba, as determined by observation and experiment, without enter-
ing in detail upon the theories of the subject. This will be followed
by a section dealing with the physical theories and physical imitations
of the movements and reactions, in the light of the facts set forth in the
first section. A brief final section will be devoted to a characterization
of the behavior of Amoeba from the standpoint of the student of
animal behavior.

I am compelled to give a full description of the normal movements
of Amoeba, as the course of the investigation showed that the prevalent
conception of these movements, on which many of the theories have
been based, is not correct.



There are few subjects that have been studied more than the nature
of the movements of Amoeba, but nothing final has been reached, even
from the descriptive standpoint. The first preliminary to an under-
standing of the nature of the movements must be to determine just what
movements take place.

The most extensive recent study of the movements of Amoeba has
been made by Rhumbler (1898), though the magnificent monograph of



the Rhizopods by Penard (1902) contains incidentally a large number
of valuable observations on this matter.

According to Rhumbler (7. c.) the movements in normal locomotion
are typically as follows : From the hinder end of the Amoeba (or of the
pseudopodium, if a single pseudopodium is under consideration) a
current of endosarc passes forward in the middle axis ; in front this flows
outward toward the sides, then backward along the surface, gradually
coming to rest. Figs. 30 and 31 , taken from Rhumbler, give diagrams
of these currents in an Amoeba moving as a whole (Fig. 30), and in
the formation of pseudopodia (Fig. 31). In an Amoeba which forms
more than one pseudopodium at once, these typical currents become
somewhat complicated (Fig. 32), but retain their main features. The
backward current shown at the sides in Figs. 30-32 is conceived to be
present also above and below, that is, over the whole surface of the
Amoeba. A diagram of the currents in side view, as given by Rhum-
bler, is shown in Fig. 33, B. An essentially similar account of the
currents is given by Biitschli (1880, 1892).

FIG. 30.*

FIG. 324

FIG. 33.

The most striking feature in the currents as above set forth is the fact
that they agree precisely with the currents produced in a drop of fluid
of any sort when the surface tension is lowered over a certain limited
area. There is always a current over the surface away from the region
where the tension is lowered, while an axial current moves toward the

*FiG. 30. Diagram of the currents in a progressing Amoeba Umax, after
Rhumbler (1898).

t FIG. 31. Diagram of the u fountain currents " in pseudopodia of Amoeba,
after Rhumbler (1898).

% FIG. 32. Diagram of complex " fountain currents" in an Amoeba with two
large pseudopodia, after Rhumbler (1898).

FIG. 33. Comparative diagrams of the currents in a rolling movement, and
in the movement of Amoeba, as conceived by Rhumbler, viewed from the side.
In A are represented what Rhumbler conceives to be the necessary currents in
a rolling movement, while B represents what Rhumbler considers the really
existing currents in Amoeba, as seen from the side. The heavier arrows in each
case represent the current on the lower surface. After Rhumbler (1898).


point of lowered tension. Diagrams of the movement of such drops
are given in Fig. 34. Further, the drop may elongate in the direction
of the axial current, and may move bodily in that direction, just as
happens in Amoeba.* It is most natural, therefore, to conclude as
Butschli (1892) and Rhumbler (1898) have done, that the movements
of Amoeba are likewise due to a lowering of the surface tension at the
anterior end, provided that its movements really take place in the
ivay described above.



At the beginning of my work I had no doubt that the movements
occurred exactly as above described, and, therefore, did not devote
special attention to this point. But I was soon struck by the fact that
I was unable to see any backward current at the sides, as represented
in Figs. 30 and 31. Further careful study of the movements of
Amoeba Umax, A. proteus, A. angulata, A. verrucosa, A. sphcero-

nucleolus, and one or two
undetermined species
confirmed this fact, and I
may say at once that after
several months' continu-

A B ous study of the move-

FIG. 34. t .

ments and reactions of

Amoeba I have never, except in one or two doubtful instances, seen
any backward movement of the substance at the sides or on the surface
of an Amoeba that was moving forward in a definite direction.

It is true that in the movements of Amceba Umax, for example, one
receives the impression of two sets of currents, one forward in the cen-
tral axis, the other backward at the sides. But if the latter is studied
carefully it is found that there is really no current here ; the proto-
plasm is at rest, and the impression of a backward current at the sides
is produced only by contrast with the forward axial current. Amceba

* All these facts are easily verified by placing a drop of clove oil on a slide in
a mixture of two parts glycerine to one part 95 per cent alcohol under a cover
supported by glass rods, as described in a previous paper by the present author
(Jennings, 1902). By mixing some soot or India ink with the clove oil the cur-
rents are made evident.

t FIG. 34. Currents in a drop of fluid when the surface tension is decreased
on one side. A, the currents in a suspended drop, when the surface tension is
decreased at a. After Berthold (1886). B, axial and surface currents in a drop
of clove oil, in which the surface tension is decreased at the side a. The drop
elongates and moves in the direction of a, so that an anterior (a) and a posterior
(p) end are distinguishable.


Umax contains usually a large number of fine granules, which in many
cases extend to the very outer surface, so that it is not possible to dis-
tinguish an ectosarc, in the sense of a layer containing no granules.
By watching the movements of these particles it is possible to determine
the direction of the currents in the protoplasm. The movements in
locomotion are usually as follows : At the anterior end there pushes
forth from the interior a clear substance, which I will call the
hyaloplasm. As this moves forward it spreads out laterally, till it
reaches a position such that it forms a continuation forward of the
remainder of the lateral boundary of the animal. Into this hyaloplasm
flows then the granular endosarc. The granules flow forward, rapidly
in the middle, usually more slowly near the sides. As it reaches the
anterior end the central current spreads out in a fanlike manner, so
that some of the granules approach closely the lateral borders of the
Amoeba (Fig. 35). They then stop, while the central part of the cur-
rent passes on, following the advancing anterior end.

So long as one confines his attention to the Amoeba alone, not
observing external objects,
one receives the impres-
sion that there are two
sets of currents, an axial
current forward, marginal
currents backward. But
as soon as one fixes his

eye upon a particular granule in the apparent backward marginal
current, and observes its relation to some external object, he dis-
covers that no such current exists. The granule remains quiet,
retaining continually its position with relation both to other granules
in the edge of the Amoeba and to objects external to the Amoeba.
Meanwhile the remainder of the substance of the Amoeba is flowing
past, so that the granule in question after a time comes to occupy
a position at the middle of the length of the Amoeba. At about
this point it usually begins to move slowly forward again, though
much less rapidly than the internal current. The nature of this
slow forward movement we shall take up later (p. 166). The main
portion of the body of the Amoeba thus continues to pass the granule,
and the latter finally reaches the posterior end. Here it usually re-
mains quiet for a time (moving forward only as the posterior end is
dragged forward). Then it is taken into the central current again,
passes to the anterior end, and comes to rest as before, while the
remainder of the Amoeba passes it by ; and this process is repeated

* FIG. 35. Diagram of the movements of particles in an advancing Amoeba.
Each broken line represents the path of a particular particle.


indefinitely. In favorable cases I have repeatedly followed a single
granule from the posterior end forward till it came to rest at the
anterior end, then watched the body of the Amoeba pass it by, until it
was again at the posterior end and started forward anew. The course

Online LibraryH. S. (Herbert Spencer) JenningsContributions to the study of the behavior of lower organisms → online text (page 36 of 50)